The present invention relates to a method for knock control in combustion.
Several knock detection methods for combustion engines are known.
Conventional methods use an accelerometer type of sensor, which sensor is mounted on the cylinder housing of the engine. By analyzing the frequency signal emitted from these sensors the typical frequencies for a knocking condition could be detected. When the intensity within the frequency range typical for a knocking conditions exceeds a certain level, then a knocking condition is established.
The disadvantage with this type of system is that other kind of vibrations, mechanical engine noise, road noise etc. , could have an impact upon the sensor output signal, and thus cause incorrect indications of a knocking condition.
In another method for knock detection a biased measuring gap is arranged within the combustion chamber. Ionization takes place within the combustion chamber during combustion, and the current developed in the measuring gap is representative for the combustion process. Similar to accelerometer type of sensors a knocking condition could be established by filtering out the superposed frequency in the ionization current typical for a knocking condition. This could be made using a system where the ordinary spark plug is used as the measuring gap, and where a substantially constant measuring voltage is applied in the grounded connection of the secondary winding of the ignition coil, as shown for example in EP,C,188.180 (=U.S. Pat. No. 4,648,367).
Knock detection using frequency analysis of the ionization current has the advantage that knock detection is not affected by mechanical engine noise, and if the ordinary spark plug is used as the measuring gap then no additional sensor on the engine is needed. Different detailed solutions for ionization current detected knock are further shown in:
U.S. Pat. No. 5,263,452, where the level of an extracted frequency component a certain time after ignition spark generation is used in order to establish a knocking condition; PA1 U.S. Pat. No. 5,005,547, where ignition timing control is initiated to find optimum timing, until a knocking condition is detected via ionization current; and PA1 U.S. Pat. No. 5,111,790, where the superposed frequency of the ionization current is detected in the primary winding of the ignition coil.
In U.S. Pat. No. 5,220,821 a method is shown where any ion current detected knock is ignored if the ion current amplitude is too low or is fluctuating. This sorting out of incorrect indications of a knocking condition, if based upon frequency analysis of the ionization current, is insensible for incorrect indications of a knocking condition developed during operating conditions when the ionization current in its normal state is stable and exceeds an acceptable signal level. This method could mainly sort out incorrect indications for a knocking condition during specific operating conditions when the ionization current is low or fluctuating, for example at low rpms and low load, respectively at other unstable operating conditions at lean or by EGR diluted air fuel mixtures.